Titanium oxide powder, methods of manufacturing the same and barium titanate powder, dielectric ceramic, and multilayer ceramic capacitor

ABSTRACT

A titanium oxide powder holding a barium compound on the surface of the particles thereof is used for manufacturing a highly crystalline fine barium titanate powder by solid-phase reaction. When the titanium oxide powder and a barium-containing powder material are mixed and calcined to prepare the barium titanate powder, the barium compound on the surfaces of the titanium oxide powder particles inhibits the sintering, or the growth, of the titanium oxide during the calcination. Consequently, the resulting barium titanate powder is highly crystalline and fine.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to titanium oxide and barium titanatepowders, methods of manufacturing the same, dielectric ceramics andmultilayer ceramic capacitors, and particularly to an improved methodfor preparing a highly crystalline fine barium titanate powder.

2. Description of the Related Art

Sintering a powder material mainly containing barium titanate results ina dielectric ceramic. The dielectric ceramic is used, for example, forforming dielectric ceramic layers included in multilayer ceramiccapacitors.

For miniaturizing the multilayer ceramic capacitors and giving them highcapacitance, it is effective to form thinner dielectric ceramic layers.In order to form thin dielectric ceramic layers, the barium titanatepowder used for the dielectric ceramic layers needs to be finer.

Hydrothermal synthesis and hydrolysis have been suggested and put topractical use to readily obtain fine barium titanate powders, but thesemethods increase the cost of manufacturing the barium titanate powders.Accordingly, solid-phase reaction has been traditionally used formanufacturing barium titanate powders.

In the solid-phase reaction, starting materials, for example, a bariumcarbonate powder and a titanium oxide powder are mixed, pulverized witha medium, and then are calcined. For manufacturing finer barium titanatepowder by solid-phase reaction, it is effective to pulverize titaniumoxide powder or to use a much finer titanium oxide powder.

Unfortunately, while barium titanate is synthesized at temperatures of1000 to 1200° C., titanium oxide is sintered at about 800° C. Hence,titanium oxide starts to be sintered, that is, grains grow, before thebarium titanate starts to be synthesized. As a result, even when thefine titanium oxide powder is used, it cannot be effective enough toobtain a fine barium titanate powder.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod of manufacturing a fine barium titanate powder by solid-phasereaction.

Another object of the present invention is to provide a titanium oxidecapable of being advantageously used in the method of manufacturing thebarium titanate powder and a method of manufacturing the titanium oxidepowder.

Still another object of the present invention is to provide a dielectricceramic prepared by sintering the barium titanate powder manufactured bythe method of the present invention and a multilayer ceramic capacitorcomprising the dielectric ceramic.

To this end, according to one aspect of the present invention, there isprovided a titanium oxide powder comprising titanium oxide particles anda barium compound on the surfaces of the particles. Specifically, thetitanium oxide powder has a layer mainly containing a barium compoundwith a predetermined thickness on the surfaces of the particles.

Preferably, the barium content of the titanium oxide powder is in therange of about 0.001 to 0.1 mol relative to 1 mol of titanium.

Another aspect of the present invention is directed to a method ofmanufacturing the titanium oxide powder described above.

The method comprises mixing a titanium oxide powder, a solvent and abarium-containing material soluble in the solvent to prepare a titaniumoxide slurry. The solvent is removed from the slurry and thesolvent-free mixture is heated so that a barium compound is present onthe surfaces of the titanium oxide powder particles.

The barium compound stabilized on the surfaces of the particles in theheating step has a thickness of about 3 to 30 nm. The barium compound isin any one of three states depending on the heating temperature and thekind of barium-containing material.

The barium compound on the surfaces of the particles may be amorphous. Arelatively low heating temperature of about 150° C. or less facilitatescreating this state.

The barium compound may in an amorphous state mainly containing BaCO₃ onthe surfaces of the particles. A medium heating temperature of about 150to 600° C. facilitates creating this state.

At least a part of the barium compound is a reaction product withtitanium oxide at the surfaces of the particles. A relatively highheating temperature of about 600° C. or more facilitates creating thisstate.

Another aspect of the present invention is directed to a method ofmanufacturing a barium titanate powder.

The method of manufacturing a barium titanate powder comprises mixing atitanium oxide powder comprising titanium oxide particles having abarium compound present on the surfaces of the particles and a powdercontaining barium to prepare a powdered mixture. Also, the powderedmixture is calcined.

Preferably, the barium content of the titanium oxide powder is in therange of about 0.001 to 0.1 mol relative to 1 mol of titanium in themethod of manufacturing the barium titanate powder.

Preferably, the titanium oxide powder has a specific surface area ofabout 5 m²/g or more, and more preferably of about 10 m²/g or more inthe method of manufacturing the barium titanate powder.

Another aspect of the present invention is directed to a dielectricceramic comprising a powder material mainly containing a barium titanatepowder prepared by the manufacturing method described above. Thematerial powder is formed into a predetermined shape and is sintered.

Another aspect of the present invention is directed to a multilayerceramic capacitor. The multilayer ceramic capacitor comprises aplurality of dielectric ceramic layers comprising the above dielectricceramic and a plurality of internal electrodes. The internal electrodesextend along predetermined interfaces between the dielectric ceramiclayers, and two opposing internal electrodes separated by one of thedielectric ceramic layers define a capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing an internal structure of amultilayer ceramic capacitor of the present invention;

FIG. 2 is a graph showing c/a axial ratios and specific surface areas ofbarium titanate powders according to Example 1 and Comparative Examples1 and 2; and

FIG. 3 is a graph showing c/a axial ratios and specific surface areas ofbarium titanate powders prepared from titanium oxide powders havingvarious barium contents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic sectional view showing an internal structure of amultilayer ceramic capacitor 1 of the present invention.

The multilayer ceramic capacitor 1 comprises a laminate 4 having aplurality of laminated dielectric ceramic layers 2 and a plurality ofinternal electrodes 3 extending along some interfaces of the dielectricceramic layers 2. The internal electrodes 3 are disposed so that twoopposing internal electrodes 3 separated by a dielectric ceramic layer 2form a capacitor.

External electrodes 5 are formed on both ends of the laminate 4 and eachexternal electrode 5 is electrically connected with alternate internalelectrodes 3; hence, the internal electrodes 3 are disposed in thelaminating direction so as to be alternately connected with one externalelectrode 5 and the other external electrode 5.

A dielectric ceramic for forming the dielectric ceramic layers 2 isprepared by forming a powder material into a sheet and by sintering thesheet. The powder material mainly contains a barium titanate powderprepared from a titanium oxide powder of the present invention.

The barium titanate powder is prepared by mixing the titanium oxidepowder of the present invention and a powder containing barium, such asbarium carbonate, and by calcining the mixture according to achieve asolid-phase reaction. The resulting barium titanate may be pulverized,if necessary.

The titanium oxide powder used for preparing the barium titanate powderin the present invention has a barium compound on the surfaces of theparticles thereof. Preferably, the titanium oxide powder contains about0.001 to 0.1 mol of barium relative to 1 mol of titanium and has aspecific surface area of about 5 m²/g or more.

The titanium oxide powder holding the barium compound on the surfaces ofthe particles thereof is prepared according to the following procedure.

A starting titanium oxide powder is mixed with a solvent and abarium-containing material soluble in the solvent to form a titaniumoxide slurry. The solvent may be water or an organic solvent, such asethanol. The barium-containing material may be barium alkoxide.

The solvent and other volatile constituents are removed from thetitanium oxide slurry by, for example, spray drying to separate out apowdered mixture.

The powdered mixture is heated in a batch furnace or the like tostabilize a barium compound, such as barium oxide, on the surfaces ofthe titanium oxide particles. The stabilized barium compound forms intoa film having a thickness of about 3 to 30 nm, and more preferably about5 to 15 nm, on the surface of each titanium oxide particle.

The inventors have ascertained that the conformation of the bariumcompound on the surfaces of the titanium oxide powder particles dependson the temperature in the heating process of the powder described above.

Specifically, when the barium-containing material is barium alkoxide andthe powdered mixture is heated at a relatively low temperature of about150° C. or less, an amorphous barium compound, particularly amorphousbarium oxide, is present on, that is, coats the surfaces of the titaniumoxide powder particles.

When the barium-containing material is barium alkoxide and the powderedmixture is heated at a medium temperature of about 150 to 600° C., anamorphous barium compound mainly containing BaCO₃ is present on, thatis, coats the surfaces of the titanium oxide powder particles. Morespecifically, crystalline BaCO₃ is present in an amorphous barium oxidephase. The carbon of the BaCO₃ is derived from CO₂ in the atmosphere,barium alkoxide and the solvent.

When the barium-containing material is barium alkoxide and the powderedmixture is heated at a relatively high temperature of about 600° C. ormore, part of the barium compound is a reaction product with titaniumoxide at the surfaces of the titanium oxide powder particles. Morespecifically, coating films mainly containing a product from a reactionof barium and titanium are formed at the surfaces of the titanium oxidepowder particles.

The resulting titanium oxide powder holding the barium compound on thesurfaces of the particles thereof can be pulverized with a dry mill orthe like, and then is used for preparing a barium titanate powder.

The titanium oxide powder holding the barium compound is mixed with apowder material containing a barium compound, such as barium carbonate,to prepare a powder mixture. If wet blending is applied for preparingthe powder mixture, the powder mixture is dried before use.

Next, the powder mixture is calcined, e.g., in a batch furnace, tosynthesize barium titanate and thus results in a barium titanate powder.In this instance, the titanium oxide powder holding the barium compoundon the surfaces of the particles thereof inhibits the sintering, thegrowth, of the titanium oxide. Thus, the resulting fine titanium oxidepowder advantageously leads to a highly crystalline fine barium titanatepowder.

Preferably, the titanium oxide powder contains about 0.001 to 0.1 mol ofbarium relative to 1 mol of titanium, as described above, to ensure theeffect of the fine titanium oxide powder.

Preferably, the titanium oxide powder holding the barium compound has aspecific surface area of about 5 m²/g or more to result in much finerbarium titanate powder with a high yield.

The resulting barium titanate powder is pulverized by, for example, adry mill if necessary.

Examples of the preparation of the barium titanate powder according tothe present invention will now be described.

EXAMPLE 1

A barium titanate powder of Example 1 is prepared according to thefollowing procedure.

First, a starting titanium oxide powder having a specific surface areaof 30 m²/g was dispersed in ethanol to form a titanium oxide slurry.Barium alkoxide was added to the titanium oxide slurry such that themolar ratio of barium to titanium was 0.02 to 1, and wet-blended. Next,ethanol was removed from the mixture by spray drying to realize a powdercontaining titanium oxide and the barium compound.

The separated powder was heated at 300° C. for 2 hours in the atmospherein a batch furnace, and then pulverized with a dry mill to obtain atitanium oxide powder of which the particles were coated with films witha thickness of 10 nm of amorphous barium oxide phases containingcrystalline BaCO₃ grains.

Next, the resulting titanium oxide powder was wet-blended with a BaCO₃powder having a specific surface area of 12 m²/g such that the molarratio of barium to titanium was 1.000 to 1.

After being dried, the powder mixture is calcined in a batch furnace,and then was pulverized with a dry mill to obtain a barium titanatepowder of the present invention.

Comparative Examples

In contrast, titanium oxide powders holding no barium components wereused for preparing barium titanate powders in comparative examples.Comparative barium titanate powders were prepared, using ComparativeExamples 1 and 2, from the titanium oxide powders having no bariumcompound and specific surface areas of 10 m²/g and 30 m²/g,respectively, using the same procedure as Example 1 except the bariumalkoxide was not used.

Calcinations for synthesizing barium titanate in Example 1 andComparative Examples 1 and 2 were each performed at various temperaturesof 1000, 1050, and 1080° C. to produce three barium titanate powdershaving different specific surface areas for each example.

The c/a axial ratios and the specific surface areas of the crystals forthe resulting barium titanate powders were measured, and the results areshown in FIG. 2.

FIG. 2 shows that c/a axial ratios of the barium titanate powders ofExample 1, in which the titanium oxide powder holding the bariumcompound on the surface of the particles thereof was used, are largerthan those of Comparative Examples 1 and 2, in which titanium oxidepowders holding no barium compound were used. Hence, the barium titanatepowders of Example 1 are highly crystalline and have large specificsurface areas, that is, have fine particles.

EXAMPLE 2

A starting titanium oxide powder having a specific surface area of 30m²/g was dispersed in ethanol to form a titanium oxide slurry. Bariumalkoxide was added to the titanium oxide slurry such that the bariumcontents were 0.0 (not added), 0.0005, 0.001, 0.02, 0.1, and 0.2 mol to1 mol of titanium, and wet-blended.

These titanium oxide slurries resulted in barium titanate powders by useof the same procedure as Example 1.

The c/a axial ratios and the specific surface areas of the crystals ofthe resulting barium titanate powders are shown in FIG. 3.

FIG. 3 suggests that titanium oxide powders holding the barium compoundcontaining less than 0.001 mol and more than 0.1 mol of barium relativeto 1 mol of titanium rarely result in highly crystalline barium titanatepowders.

Hence, the barium content of the titanium oxide powder is preferably inthe range of about 0.001 to 0.1 mol relative to 1 mol of titanium.

According to the present invention, a titanium oxide powder holding thebarium compound on the surface of the particles thereof is provided. Thebarium compound may be amorphous, an amorphous state containing BaCO₃ ora product from a reaction with titanium oxide. When the titanium oxideholding the barium compound is used to synthesize barium titanate bysolid-phase reaction and thus results in a barium titanate powder, thebarium compound on the surfaces of the titanium oxide powder particles,which covers the surfaces of the particles, inhibits the sintering, orthe growth, of titanium oxide. Thus, the resulting fine titanium oxidepowder advantageously results in a highly crystalline and fine bariumtitanate powder at lower cost than those made by hydrothermal synthesisand hydrolysis.

By preparing the titanium oxide powder so as to contain about 0.001 to0.1 mol of barium per mol of titanium, the effect of the fine titaniumoxide powder can be ensured.

By using the titanium oxide powder holding the barium compound having aspecific surface area of about 5 m²/g or more for preparing the bariumtitanate powder, much finer barium titanate powder can be obtained witha high yield.

In addition, by producing multilayer ceramic capacitors comprising adielectric ceramic formed by sintering the materials mainly containingthe barium titanate powder according to the present invention, thereliability of the multilayer ceramic capacitors is ensured even thoughdielectric ceramic layers become thinner in order to miniaturize themultilayer ceramic capacitors.

Various changes and modifications can be made in the process andproducts of this invention without departing from the spirit and scopethereof. The various embodiments disclosed herein were for the purposeof illustration only and were not intended to limit the invention.

What is claimed is:
 1. A titanium oxide powder comprising titanium oxideparticles having a layer of a barium compound of predetermined thicknesson the surfaces of the particles.
 2. A titanium oxide powder accordingto claim 1, wherein the barium content thereof is in the range of about0.001 to 0.1 mol per mol of titanium.
 3. A titanium oxide powderaccording to claim 2, wherein the specific surface area thereof is about5 m²/g or more.
 4. A titanium oxide powder according to claim 3, whereinthe specific surface area thereof is about 10 m²/g or more.
 5. Atitanium oxide powder according to claim 1, wherein the barium compoundon the surfaces of the particles is amorphous.
 6. A titanium oxidepowder according to claim 1, wherein the barium compound on the surfacesof the particles is in an amorphous state and comprises BaCO₃.
 7. Atitanium oxide powder according to claim 1, wherein at least part of thebarium compound at the surfaces of the particles is a reaction productwith titanium oxide.
 8. A titanium oxide powder according to claim 1,wherein the specific surface area thereof is about 5 m²/g or more.
 9. Atitanium oxide powder according to claim 1, wherein the specific surfacearea thereof is about 10 m²/g or more.
 10. In a multilayer ceramiccapacitor comprising a plurality of dielectric ceramic layers; and aplurality of internal electrodes extending along predeterminedinterfaces between the dielectric ceramic layers, wherein two opposinginternal electrodes separated by one of the dielectric ceramic layers todefine a capacitor, the improvement which comprises the dielectriccomprising a sintered barium titanate according to claim
 1. 11. Atitanium oxide powder according to claim 1, wherein the layer of bariumcompound has a thickness of about three to thirty μm.